1. Field of the Invention
The present invention relates to the formation of shimming bodies for use in a nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) system. More particularly, the invention relates to an improved and simplified method for forming passive shimming bodies. The shimming bodies are for use in a superconducting magnet assembly for magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d).
2. The Prior Art
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment. For example, the magnet can be enclosed in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting. The current introduced by a power source will continue to flow through the coils even after power is removed. This continuance of current flow is due to the absence of resistance, and a strong magnetic field is maintained thereby. Superconducting magnets find wide application in the field of MRI.
To compensate for inhomogeneities in NMR or MRI magnets, various arrangements including passive ferromagnetic shim materials have been used. For example, U.S. Pat. No. 5,003,266 to Palkovich et al., U.S. Pat. No. 5,168,231 to Aubert, U.S. Pat. No. 5,923,235 to Van Oort, U.S. Pat. No. 5,999,076 to Becker, Jr. et al. and U.S. Pat. No. 6,218,838 to McGinley et al. disclose passive shimming systems for magnetic resonance magnets.
Open architecture MRI magnets tend to produce increased field inhomogeneity due to increased coil deformation and coil misalignment. Such inhomogeneity is minimized during the design stage, and then passive shim systems are added. The passive shim systems reduce the inhomogeneity that remains after the manufacturing cycle due to manufacturing tolerances and design restrictions. The passive shims are positioned between the imaging bore and the gradient coil. For example, rails having shim disks in pockets may be spaced around the magnet bore axis.
NMR shimming requires that a specific amount of ferromagnetic material be installed in a specific location. The amount varies by location due to magnetic material in the vicinity of the magnet at the installation site. The current method installs pellets in a preformed disk to obtain the desired amount of ferromagnetic material in the disk. The disks, however, must be manually and laboriously populated to produce the amount of magnetic material in each disk. A second drawback to this approach is that the pellets are added in relatively large increments making exact shimming difficult. A third drawback is that the cost of the populated disks is relatively high.
Accordingly, a method of forming a shimming body is needed that will eliminate the laborious pellet loading. A method is also needed that will eliminate the relatively incremental nature of the xe2x80x9cpellets in disksxe2x80x9d method. Also needed is a method that will lower the cost of the shimming disk.
A method of forming a shimming body is provided. In accordance with one form of the invention, a magnetizable metal powder, such as a ferrous powder, of known magnetic properties is provided. The powder is uniformly dispersed into a non-magnetic or non-ferrous material. A mixture is formed having a selected uniform density. A desired weight of magnetic material is selected for a particular installation of an NMR assembly. The mixture is heated. A selected volume of the mixture corresponding to the selected weight of magnetic material is extruded into a container. The mixture is then cooled to form a shimming body.
In another aspect, a method of forming a passive shimming disk for an open MRI assembly is provided. In this aspect, a neodymium-boron-iron alloy powder of known magnetic properties is provided. The powder is uniformly dispersed into a thermoplastic material to form a mixture having a selected uniform density. A weight of magnetic material in the mixture is determined for a particular installation of an MRI assembly. The mixture is heated. A selected volume of the heated mixture corresponding to the selected weight is extruded into an open U-shaped container. The container may be formed from a thermosetting plastic. The mixture is then cooled to form a shimming disk or body.
In another aspect, the cooled mixture is removed from the container to form the shimming disk or body.
In a further aspect, the container has a bottom and sides connected to the bottom formed from a thermosetting plastic. The mixture is cooled to form the shimming disk or body.